Full Package upgrade instructions: Reorder file deletion

The description of the files that need to be deleted should be in alphabetical order, to reduce the chance of erroneous deletion

The "files/" entry need to be moved up one place to accomplish this

For context: I have erroneously deleted the files/ directory in the last two updates, and there are surely more than be that to that mistake

https://www.phpbb.com/support/docs/en/3.3/ug/upgradeguide/update_full/